A steam generator of the type used in a pressurized-water reactor power plant, commonly comprises a vertical casing having a feed-water inlet and an upper portion of enlarged diameter which on its top forms a steam dome having a steam outlet. In the lower portion of the casing a tube plate forms a closure of the casing, a U-tube bundle having upwardly extending hot and cold legs being mounted in this tube plate, the casing below the tube plate forming a primary header having primary fluid hot and cold nozzles and a partition for directing primary fluid from the hot nozzle through the hot leg and from the cold leg through the cold nozzle. With the reactor's pressurized-water coolant forming the primary fluid, the coolant is delivered from the reactor to the steam generator's hot nozzle, passes through the hot leg of the U-tube bundle and, with a substantial amount of heat removed, goes down through the cold leg and out through the cold nozzle for return to the reactor.
The tube bundle within the casing is circumferentially enclosed by a shroud, the shroud's outside and the inside of the casing being radially interspaced and forming a descent space, the bottom of the shroud being spaced above the tube plate so that the descent space is in flow communication with the inside of the shroud containing the tube bundle. The top of the shroud mounts a group of water separators which extend horizontally above the horizontal extent of the tube bundle, including both of its legs, these water separators being under the steam dome and functioning to separate steam-water mixtures rising from the tube bundle, so that steam is passed into the steam dome while the separated water is guided to the descent space.
The feed water is normally maintained at a level above the top of the tube bundle and, of course, below the tops of the water separators from which the steam discharges. Usually steam dryers are positioned above the water separators and within the steam dome, so that the generator output is dried steam.
The casing of the steam generator is normally cylindrical, the tube bundle shroud is correspondingly cylindrical and the tube bundle itself is generally cylindrical as to its outside contour, each of the two legs having flat interfacing contours forming a vertically extending space or corridor between them which extends up to the tube bends at the top of the tube bundle. The group of water separators are horizontally arranged to form what is, in effect, a cylindrical group with the water separators uniformly interspaced and positioned substantially or approximately concentrically on top of the shroud and with respect to the tube bundle. The various parts are generally symmetrical throughout with respect to each other.
It is desirable that all of the water separators be uniformly loaded during the operation of the steam generator. However, it has been found that because of its higher temperature the hot leg produces steam at a greater rate than the cold leg, the consequence being that the loads under which the water separators positioned above the hot leg must operate, are substantially higher than the load under which the other separators above the cold leg, must operate, even though each of the two legs of the tube bundle are necessarily the same in number and in pitch, because the tube bundle is made up of the individual U-tubes having the return bends.